Ensure empty mixed pools also get saved to config/presets

[CovenEsme]

What does this address?

Fixes the issue where loading a preset with mixed pools will only overwrite the non-empty pools.

E.g. if you have a preset where: mixed_entrance_pools: [[Dungeon, Door]]

And you load that preset over existing settings where: mixed_entrance_pools: [[Door], [Overworld], [Trial Gate]]

You would get this: mixed_entrance_pools: [[Dungeon, Door], [Overworld], [Trial Gate]]

instead of: mixed_entrance_pools: [[Dungeon, Door]]

How did/do you test these changes?

• Add something to mixed pool 1
• Save that as a preset
• Add something to mixed pool 2
• Load the saved preset
• Mixed pool 2 should be empty (and was when I tested it)